This invention relates to the cooling of high temperature surfaces and particularly to the air jet impingement cooling of hot surfaces in a turbomachine aircraft engine.
The gas temperatures occurring in turbojet engines usually exceeds the temperature capabilities of the materials used for engine construction by a significant margin. The successful use of metals and other available materials in such engines therefore necessitates the precise cooling of elements in the hot section of the engine in order to avoid structural failures. Generally the cooling or temperature control of such elements is accomplished by a number of heat transfer arrangements including film and convection cooling--usually applied to both the heat exposed and heat source opposite sides of a heated member respectively.
An effective means for accomplishing convection cooling is found to reside in the technique of impingement cooling--an arrangement wherein a series of pressurized fluid jets are impinged on the reverse or heat source opposite side of the metal walls exposed to hot gas flow. Impingement cooling is therefore commonly used in the temperature control of hot surfaces in the rear portion of a turbojet engine. When impingement cooling is combined with film cooling that is located on the front or heat source side of these metal surfaces, a desirably effective total cooling arrangement is achieved. Such cooling is capable of accomplishing the needed control of metal temperatures, even with a relatively small flow of cooling fluid. The cooling fluid in such engines is usually air drawn from a cold section of the engine, such as from one of the compressor stages, for example. The relatively small flow of air needed in these cooling arrangements therefore offers a desirably small penalty on engine performance.
Heretofore the structural unity of an impingement cooling baffle and the element being cooled, has presented structural problems relating to thermal expansion and contraction of the involved materials between the temperature extremes encountered in engine operation. The close physical relationship of these two component elements in an engine and the widely varying temperature environment imposed frequently results in the occurrence of fatigue cracking and other forms of structural degradation which are undesirable in operating equipment such as aircraft.
The patent art includes several examples of cooling structures that are used in turbomachines such as aircraft engines and in other multiple layer heated environments. An example of these prior structures is found in the patent of Terry T. Eckert et al, U.S. Pat. No. 4,071,194, which concerns an arrangement for cooling the sidewalls of an exhaust nozzle using combined boundary layer or film cooling and impingement cooling. The Eckert et al invention is also concerned with thermal dimension changes occurring in high temperature structures and accommodates these dimension changes though the use of sliding mounts or slip joints which provide movement freedom in the lateral direction--as an accommodation response to thermally induced element dimension changes.
Other cooling arrangements for hot engine associated members is shown in the patent of Clayton G. Coffey et al, U.S. Pat. No. 4,355,507. The Coffey et al invention involves film cooling applied to a hot exhaust gas conducting conduit and seeks to maintain low temperatures in the cooled members in order to suppress the infrared emissions or infrared signature of the cooled object.
Another example of previous cooling arrangements is found in the patent of Joel F. Sutton et al, U.S. Pat. No. 4,081,137, which concerns the use of a corrugated structure for the rear nozzle portion of a jet engine. The Sutton et al apparatus employs an array of the corrugated elements to supply cooling air to the heated nozzle surfaces. The Sutton et al apparatus is also concerned with cooling air leakage and the maintenance of cooling in the presence of nozzle element movement.
Another example of aircraft engine cooling arrangements is found in the patent of Michel R. Jannot et al. U.S. Pat. No. 3,848,697, which also concerns the use of corrugated elements, elements that are disposed around the periphery of a turbojet engine rear portion in order to achieve acoustic damping of the engine exhaust stream and to provide cooling. Alternate corrugations of the Jannot et al structure are used for cooling air and engine exhaust containment. The Jannot et al invention involves the use of Helmholtz resonator principles in a variety of open-ended and closed-ended corrugation structures disposed surrounding a perforated exhaust gas enclosure. FIG. 8 of the Jannot et al patent also shows the use of a corrugation of increased height or amplitude as a separator between the hot engine liner and an external enclosing surface.
An example of controlled heat transfer using a quilted like or dimpled pattern in a metal foil insulating layer is found in the patent of John Jenkinson, U.S. Pat. No. 3,982,850. In the Jenkinson invention the heat transfer to an engine structural member and thereby the operating temperature and physical size of the structural member are controlled by covering the member with dimpled foil and by spacing the location of spot welds between the foil and the structural member at heat transfer determined intervals. Relative expansion movement between the dimpled foil and the structural member is contemplated in the Jenkinson invention notwithstanding the presence of the heat transfer spot welds. The use of a row and column organized dimple pattern is shown in FIG. 3 of the Jenkinson patent. Use of spot weld attachment disposed over the foil surface, concern with temperature tracking rather than element cooling, and the absence of teaching regarding thermal stress and fatigue effects are notable distinctions between the Jenkinson patent and the present invention.
While each of these prior patents contributes to the overall state of the engine element cooling art and to the structural arrangements used in engine cooling devices, none of these patents affords the cooling and structural advantages of the present invention apparatus, as will be understood from the following description.